To improve high temperature stability over amine cured epoxy resin systems and to give better physical and electrical properties, it has been the general practice in the epoxy technological field to use anhydride curing agents with epoxy resins. Most anhydride formulations require elevated-temperature cures, and for most commercial applications, it is necessary to add some form of catalyst to speed the rate of cure. The catalyst is preferably a latent catalyst which would provide rapid cure of the epoxy resin at about 135.degree.-180.degree. C., and yet not harm the storage life of the resin. In practice, the viscosity of the resin should not increase over about 1000 cp at room temperature after periods of one to several months.
Several latent catalysts have appeared on the commercial scene in recent years. Included are benzyldimethylamine, methyl imidazole, stannous octoate, benzyldimethyl aniline, "extra-coordinate" siliconate salts, triethanolamine borate, triethanolamine titanate and various other metal chelates. These type catalysts provide very fast gel times and are particularly suitable for potting compositions employing less reactive epoxy resins such as aliphatic epoxides. Other type catalysts, more suitable for impregnating varnishes made from bisphenol A or novolac epoxides, would include quaternary ammonium halides such as benzyltrimethylammonium chloride. Dante et al., U.S. Pat. No. 3,547,885, taught improved latent catalysts in the form of tetraphosphonium halide catalysts for aromatic and aliphatic dicarboxylic acid cured epoxy systems and Smith, in U.S. Pat. No. 3,784,583, assigned to the assignee of this invention, taught quaternary organic phosphonium propionates, acetates, butyrates, isobutyrates and dimethyl phosphates as particularly effective latent catalysts for anhydride cured epoxy systems.
All of these latent catalysts, while providing good gel times, leave much to be desired in terms of storage properties for the epoxy-anhydride system. What is needed is a non-catalytic chemical stabilizer, that can be used with the latent catalyzed epoxy-anhydride system, which would improve the pot life of the system.